Panels can be created in a non-panel design by leaving non-zero-width webs between regions. This is manufacturable as such (well, given some more considerations, i.e., use actual mousebites or whatever). If you like, the webs can be made zero width by also cutting them with a Board Cutout Region of the same dimension.
There are three board areas in this design; they are joined by thin traces which draw the board outline. Two such traces and one cutout region are selected, and (partially) shown in List for reference.
Somewhat surprisingly, 3D view shows no web between them, but apparently the side walls correctly cancel each other out.
To create this outline, select the outermost perimeter, D, S, D (Design / Board Shape / Define from selected objects), then place board cutouts in the remaining regions (here, the wide areas between outline sides).
As you can see, the fabricator figured out the correct intent given the data and labeling, and chose an arrangement suitable for their process (as instructed).
(Disclosure: the 7-transistor is my logo.)
I don't know if this is the kind of thing you're going for specifically, but it shows one realization (from EDA design to fabrication) of the technique.